Constant velocity universal joints are incorporated into power transmission mechanisms of an automobile or industrial machinery of various types for use. The constant velocity universal joints include two shafts on a drive side and a driven side that are provided to have a variable crossing angle, and couple the two shafts so as to transmit rotation torque from the drive side to the driven side at constant velocity. The constant velocity universal joints are roughly classified into a fixed type constant velocity universal joint that allows only angular displacement and a plunging type constant velocity universal joint that allows angular displacement and axial displacement, and are chosen as appropriate depending on their uses.
For example, as the fixed type constant velocity universal joint, a Birfield joint (BJ) and an undercut-free joint (UJ) are known. The fixed type constant velocity universal joint of those types includes: an outer member (long stem type) including a cup section in which a plurality of guide grooves extending in an axial direction are formed in a spherical radially-inner surface, and including a shaft section having one end coupled to a bottom portion of the cup section and the other end provided with a spline; an inner member in which a plurality of guide grooves extending in the axial direction in pairs with the guide grooves of the outer member are formed in a spherical radially-outer surface; a plurality of torque transmitting balls disposed in ball tracks formed by cooperation of both the guide grooves of the outer member and the guide grooves of the inner member; and a cage disposed between the outer member and the inner member, for retaining the torque transmitting balls (Patent Literature 1 and Patent Literature 2).
As the plunging type constant velocity universal joint, a double offset joint (DOJ) and a tripod joint (TJ) are known. The plunging type constant velocity universal joint of the double offset type includes, as main components: an outer member (long stem type) including a cup section in which a plurality of track grooves extending in the axial direction are formed in a cylindrical inner peripheral surface, and including a shaft section having one end coupled to a bottom portion of the cup section and the other end provided with a spline; an inner member in which track grooves extending in the axial direction in pairs with the track grooves of the outer member are formed in a spherical outer peripheral surface; a plurality of balls interposed between the track grooves of the outer member and the track grooves of the inner member, for transmitting torque; and a cage interposed between the cylindrical inner peripheral surface of the outer member and the spherical outer peripheral surface of the inner member, for retaining the balls.
The outer member of the long stem type of the constant velocity universal joint described in Patent Literature 1 is subjected to heat treatment after a part forming the cup section and a part forming the shaft section are joined to each other. The spline (serration) is formed in the outer peripheral surface at one end in the axial direction of the part forming the shaft section, to thereby achieve a reduction in the number of parts, and a cost reduction attained by a reduction of joining portions. In addition, only the joining portion is increased in diameter, and hence strength of the joining portion is increased. Further, the strength can be increased by performing the heat treatment on substantially the entire region in the axial direction of the part forming the shaft section. Accordingly, it is possible to thin the shaft section, and to achieve a reduction in weight.
Further, Patent Literature 2 describes the joining shaft used as a drive shaft and the like. As a material of the joining shaft, steel basically containing carbon steel to which at least Cr of 0.2 to 0.8 wt % is added as an element improving hardenability is used, and the joining shaft joins the shafts to each other by friction welding. The friction welding is one kind of direct solid-state welding method of joining solids to each other without melting the solids, specifically, a joining method of rubbing members to be joined (for example, a metal, a resin, etc.) against each other at high speed and applying pressure simultaneously with softening the members by frictional heat generated by the rubbing. The friction welding method is advantageous in obtaining a product having high dimension accuracy with relatively easy work, and in performing assembly and joining of members subjected to finishing. In the joining portion subjected to the friction welding, a bainitic abnormal structure is generated. However, the joining portion is subjected to partial heating and hardening, to thereby be transformed into an austenitic structure. Then, the joining portion is quench-hardened to its center by cooling performed after the heating, and thus strength of the joining portion is increased to be higher in strength than other portions, with the result that the joining portion has reserve strength.